Reduced Bimolecular Charge Recombination Loss in Thermally Annealed Bilayer Heterojunction Photovoltaic Devices with Large External Quantum Efficiency and Fill Factor

Thermally annealed bilayer heterojunction was shown to be an alternative approach to form the bulk-heterojunction (BHJ) structure and yield superior device performance compared to blend-solution-processed BHJ organic photovoltaic (OPV) devices. The best poly(3-hexylthiophene) (P3HT)/[6,6]-phenyl-C61-butyric acid methyl ester (PCBM) based bilayer structure OPV showed an external quantum efficiency approaching 82%, a high fill factor of 74%, and the power conversion efficiency of 5.1%, which are higher than those of the blend-solution-processed BHJ OPV devices. In contrast to the BHJ formed from blend solution, richer acceptor, e.g. PCBM, close to the cathode, or different morphology, was generally found in thermally annealed bilayer films. However, the physical mechanism for improved performance in thermally annealed bilayer films was rarely investigated in previous studies. Here, the amount of long-lived photogenerated charges with lifetime longer than 1 mu s, which was characterized by the time delayed collection field method, was slightly enhanced by about 5% in the thermally annealed bilayer devices. Nevertheless, the bimolecular recombination coefficient was strongly decreased by 350% with transient photovoltage/photocurrent measurements, conclusively demonstrating that the significantly reduced bimolecular recombination loss is a major contribution to the increased efficiency in the thermally annealed bilayer OPVs.